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LM5088-2 VERY large switching frequency noise

I have a custom board using the LM5088MHX-2/NOPB to generate a -20V @ 1A and a second to generate -15V @1A.  These are also used in conjunction with some positive output supplies that share a ground.  

I have a lot of noise, and have turned off all supplies except the -15V supply to isolate the noise.  On the output of the -15V supply there are spikes near the switching frequency of several volts in magnitude.  I used values straight out of webbence, however can't seem to get webbench to give me a schematic again with this part.  Any ideas what might cause this?  The noise is large enough that it seems something is connected very wrong.  I've attached a snapshot of the schematic.  The input is +24V from a battery.  

  • Hi Ryan,

    For WEBENCH design creation issue, please try again after clearing your browser cache. I was able to create a LM5088-2 Buck-Boost design with this set of input conditions: 20-26Vin, -15Vout and 1A Iout.

    For the issue of large switching noise, it's difficult to pinpoint the issue without seeing your layout or some waveforms. Also, there is a chance it may need a snubber circuit at the switching node.
  • Hi Ryan,

    Thanks for using WEBENCH. As mentioned in above post, there could be variety of reasons for ringing noise and voltage spikes, such as; board layout, wiring, etc . Among those, sometimes measurement setup itself can cause large spikes and bad reading. Are you using wrapped leads for your measuring probe? Are you using the eval board layout which is modified for the inverting topology?

    Thanks,

    Hadi

  • Thanks for the response.  I was assuming that since the voltage spikes were so large there was a schematic or other problem.  However, based on your response I think the schematic is ok, just have some major layout problems.  I have tried using wrapped leads for measurement, as well as standard scope probes.  I don't think these volts of spikes are due to the probes.  I think the spikes are real, just need to look at the layout again.  Thanks.  

  • Hey..I am using a very similar design. But I have problems designing the snubber circuit.Can u help me by providing the values for Rsnub and Csnub? or how to calculate it using switching frequency?..My switching frequency is 550kHz.